The present invention relates to open-center fluid controllers of the type used to control the flow of fluid from a source of fluid to a fluid pressure operated device, such as a steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter and valving, and an arrangement for imparting follow-up movement to the valving in response to flow through the fluid meter. In an open-center controller, the flow through the controller is not directly proportional to a main variable flow control orifice, and to the deflection (displacement) of the valving. Instead, the deflection of the valving in an open-center controller depends upon the load, as represented by the pressure drop across the controller.
It has long been an object of those skilled in the art to provide a steering system, including a fluid controller, in which the total flow through the steering system is substantially greater than the flow through the controller, but with the overall system flow being related to the flow through the controller, in a known manner. See, for example, U.S. Pat. No. 4,052,929 in which the controller receives fluid from one pump then generates a pilot signal to control a pilot operated valve which receives fluid from a second pump. The total steering flow comprises the flow through the pilot operated valve, plus the flow from the controller. Such a system is theoretically satisfactory, but the cost of such a system becomes nearly prohibitive because of the addition of the pilot operated valve and the second pump.
U.S. Pat. No. 4,759,182, assigned to the assignee of the present invention, and incorporated herein by reference, discloses a load-sensing (closed-center) fluid controller in which the valving defines an amplification fluid path, including a variable amplification orifice in parallel with the main fluid path. In the preferred embodiment of the device of the above-incorporated patent, the amplification fluid path is in fluid communication with the main fluid path at a first location disposed between the fluid inlet port and a first variable flow control orifice, typically referred to as the main variable flow control orifice, or simply the A1 orifice. As is well known to those skilled in the art, in load-sensing controllers, the pressure drop across the A1 orifice is maintained substantially constant, and therefore, the flow through the A1 orifice, and the controller, is directly proportional to the size of the A1 orifice.
In open-center controllers of the type to which the present invention relates, however, there is no main variable flow control orifice. Instead, in an open-center controller, there is a constant flow of fluid through the controller valving to the reservoir when the controller is in neutral, and the pressure of this fluid flow is inherently just slightly above the pressure in the reservoir. As the valving is displaced from neutral, the neutral flow control orifice to the reservoir begins to close and build pressure, and the operator continues to displace the steering wheel and the valving until the fluid pressure builds to a level sufficient to overcome the load on the steering cylinder.
On many of the vehicles which utilize open-center controllers, it is also desirable for the controller to provide a manual steering capability, i.e., the ability to generate pressurized fluid by rotation of the steering wheel, valving, and fluid meter when the pump is inoperative, or for some other reason is unable to generate fluid pressure. It has been discovered that when the amplification fluid path of U.S. Pat. No. 4,759,182 is applied to fluid controllers, any attempts to manually steer the vehicle tend to be unsuccessful.